<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        /* .box{
      width: 200px;
      height: 200px;
      background-color: red;
      clip-path: polygon(0 0, 200px 0, 0 200px);
    } */
        /* .box{
      width: 0px;
      height: 0px;
      border: 50px solid #000;
      border-top: 50px solid #000;
      border-right: 100px solid #000;
      border-bottom: 50px solid #000;
      border-left: 100px solid #000;
      border-top-color: transparent;
      border-right-color: transparent;
      border-bottom-color: transparent;
    } */

        .box {
            width: 0px;
            height: 0px;
            border: 50px solid #000;
            border-radius: 80%;
            border-top-color: transparent;
            border-right-color: transparent;
            border-bottom-color: transparent;
            /* clip-path: polygon(0 0, 100px 0, 50px 50px); */
        }
    </style>
</head>

<body>
    <div class="box"></div>
</body>

</html>